The present invention relates to the structure of an optical write head using light-emitting device arrays to be provided in a high-resolution electrophotographic printer, and to a method of assembling the optical write head.
An optical write head to be used in an optical printer has hitherto been equipped with light-emitting device arrays, such as light-emitting diodes. The principle of an optical printer equipped with an optical write head is illustrated in FIG. 9. The surface of a cylindrical photosensitive drum 2 is coated with material (photosensitive material) possessing optical conductivity, such as amorphous Si or organic material. The photosensitive drum 2 rotates in accordance with a print speed. To begin with printing, the surface of photosensitive material provided over a rotating drum is charged uniformly with an electrostatic charger 4.
Next, an optical write head 6 radiates, onto the photosensitive material, light which assumes the image of a dot to be printed, thereby neutralizing the thus-exposed portion of the photosensitive material and forming a latent image. Subsequently, a development unit 8 causes toner to adhere to the photosensitive material in accordance with the charged status of the photosensitive material. A transfer unit 10 transfers toner onto paper 14 supplied from a cassette 12. A fixing unit 16 applies heat to the paper, thereby fixing the toner transferred on the paper. The paper is then fed to a stacker 18. After transfer of the latent image has been completed, the entirety of the electrically-charged photosensitive drum 2 is neutralized by an erasure lamp 20, and residual toner is removed by a cleaner unit 22.
The optical write head which has hitherto been employed is constructed such that a plurality of light-emitting device array chips are arranged in a single line or in a staggered layout on a substrate, in accordance with specifications pertaining to a print width and such that a rod-lens array or rod-lens arrays (e.g., product name: SELFOC Lens array manufactured by Nippon Sheet Glass Co. Ltd.) having gradient index rod lenses stacked thereon in the form of one or two lines is arranged opposite the light-emitting device array chips. FIG. 10 is a perspective view showing a rod lens array 11 having rod lenses stacked in two rows. A plurality gradient index rod lenses 24 are sandwiched between frames 26 and secured by means of resin 28.
FIG. 11 is a cross-sectional view showing a typical example of an optical write head constructed as described above. In this example, a plurality of light-emitting device array chips 30 are arranged in a single line on a printed circuit board 32 formed from glass epoxy, in accordance with specifications pertaining to a print width. A rod-lens array 24 having gradient index rod lenses stacked thereon in the form of one or two lines is arranged opposite the light-emitting device array chips 30. The light-emitting device array chips 30 and the rod lens arrays 34 are fixed on a housing 36 by means of a silicon filler 38.
(I) In association with an increase in print speed and an improvement in resolution, required precision of alignment of an optical system is increased significantly. A geometrical layout or related-art mechanical components fails to maintain the precision of the mechanical components, to thereby fail to satisfy performance requirements of the optical system.
Obtaining a high-resolution image requires setting within a range of xc2x130 xcexcm respective specified values of the amount of deviation between the center of an optical axis and an illumination point of each or light-emitting devices, the distance from the illumination point to the end face of a rod lens array, and the distance from a photosensitive surface to the end face of the rod lens array.
(1) The lengths of rod lenses have a variation of about xc2x10.4 mm in manufacture. (2) There may be a case where the rod lens array has warpage toward an image surface or an object surface. (3) Fiber-reinforced plastics (FRP) from which a frame of the rod lens array is formed has a variation of about xc2x10.4 mm in thickness. Even if an optical component is positioned along the mechanical components, the precision of positioning exceeds a required range of optical precision, thereby failing to satisfying optical performance.
For this reason, there has arisen a necessity of three-dimensionally aligning the position of the rod lens array to a light-emitting device array. More specifically, (1) the distance between an illumination point and the surface of a photosensitive material must be matched with a conjugate length of the rod lens array; (2) the longitudinal center of a lens belonging to the rod lens array must be set to the center of the distance; and (3) deviation between the optical axis of the rod lens array, an illumination point, and the position of the surface of a photosensitive material must be adjusted with respect to the longitudinal direction of the rod lens array.
For this reason, a space is ensured beforehand between the housing of the optical write head and the rod lens array. The rod lens array is three-dimensionally aligned. The rod lens array is secured on the housing of the optical write head by means of filling the space with a silicon-based adhesive.
However, in order to align the optical axis of the rod lens array in the longitudinal direction thereof, the alignment must be performed through use of an actuator having high positional precision. An enormous amount on alignment time is required.
The mechanical components must be formed into complicated shapes, by means of ensuring, for example, a space for effecting alignment of an optical axis. Such complicated working of the mechanical components is a contributory factor to hindering a reduction in manufacturing costs of an optical write head.
In most cases, a head housing of the related-art optical write head is molded from engineering plastics. In a case where a rod lens array having the optical axis aligned is secured to the head housing, a silicon-based adhesive is usually used. Heat contraction (a contraction of about 8% arises in volume of the adhesive), which arises after the adhesive has been tilled and cured, or distortion of material of the head housing, which is caused by contraction of the head housing with time, poses difficulty in guaranteeing the positional precision of the optical axis over a period of years.
The present invention has been conceived to solve the drawbacks set forth and is aimed at providing an optical write head which obviates a necessity of alignment operation by use of a high-precision device and a necessity of complicated mechanical components, enables lower-cost manufacture of the optical write head, and is less susceptible to time-varying changes.
(II) In principle, in a primary scanning direction (i.e., the direction in which light-emitting points are to be scanned; that is, a direction perpendicular to a sheet of FIG. 11), the optical write head 6 having the light-emitting device array chips 30 arranged thereon must be made greater in size than a print width. In order to reduce the overall size of a printer using the optical write head 6, demand exists for reducing the size of the printer in a sub-scanning direction (i.e., a direction in which the photosensitive drum 2 rotates) As shown in FIG. 11, the printed circuit board 32 having the light-emitting array chips 30 mounted thereon must be arranged perpendicular to a light-emission optical axis 39. For this reason, a reduction in the width of the substrate 32 is effective for reducing the dimension of the printer in the sub-scanning direction.
A light-emitting diode (LED) array is commonly and widely used as a light-emitting array. Supply of a signal corresponding to an image signal output from a driver integrated circuit (IC) to LED chip arrays requires formation of bonding pads (BPs) equal in number to LED devices on the LED array chip 30. In the case of a resolution of 600 dpi, a pitch at which LED devices are to be arranged is 42.3 xcexcm. Provided that a side of an area in which bonding pads (BP) are arranged is 80 xcexcm, a pitch at which BPs are to be arranged is 80 xcexcm or more. At least two rows of BPs must be arranged in the direction parallel to the direction in which LEDs are arranged.
In a case of manufacture of a print head of 600 dpi for A3-size paper, light-emitting points to be arranged in a one-dimensional layout assume a number of 7,000 or more. Wire bonds (WB) equal in number to the light-emitting points must be connected to a driver IC. Hence, driver IC chips 31 are die-bonded to the substrate on which the light-emitting array chips 30 are mounted. The driver IC chips 31 are connected to the light-emitting array chips 30 through use of an Au line 33 and by means of wiring bonding.
A driver IC chip must be disposed on either side of an LED array chip having a high density of light-emitting points for use with a high-definition printer. For this reason, difficulties are encountered in reducing the width of the substrate 32 to a certain extent or more. The substrate 32 on which the driver IC chip 31 is disposed on either side of the light-emitting array chip 30 usually assumes a width of about 12 mm to 20 mm.
A space of 5 mm to 10 mm width is required for mounting connectors for drawing wires from the substrate 32 or for soldering a flexible printed circuit film or sheet.
In order to prevent an increase in the width of the substrate 32, which would otherwise be caused by ensuring a wiring space, the related-art technique has hitherto employed a method of elongating a substrate in a primary scanning direction and mounting connectors in a range on the substrate where no light-emitting device array chips are to be present, through use of through holes; a method or mounting connectors on the reverse surface of a substrate by means or surface mount technique; or a method of mounting a flexible printed circuit film or sheet on the reverse side of a substrate by means of soldering.
In order to reduce the number of wires to be bonded to the LED arrays, inventions have been proposed [Japanese Patent Application Laid-Open Nos. 238962(1989), 14584(1990), 92650 (1990), and 92651 (1990)], wherein light-emitting thyristors of a p-n-p-n structure are adopted as constituent elements of the light-emitting array, thereby enabling self-scanning or light-emitting points. The inventions describe the ability to facilitate mounting of light-emitting thyristors as a light source for an optical printer, to reduce an area within which light-emitting devices chips are to be mounted, and to manufacture a compact light-emitting device.
Further, an invention has been proposed [Japanese Patent Application Laid-Open No. 263668(1990)], in which a switching device array is taken as a transfer section and is isolated from a light-emitting device (i.e., light-emitting thyristor) array.
FIG. 12 shows an equivalent circuit of the self-scan-type light-emitting array. The light-emitting device is made up of an array of transfer thyristor devices T(1), T(2), . . . and light-emitting tyristors L(1), L(2), . . . The drawing shows only a portion of the array. The transfer thyristor devices T(1), T(2), . . . are connected by means of diodes D1, D2, . . . VGA denotes a power line (usually assuming xe2x88x925V ) which is connected to a gate electrode of each of the thyristor devices T and L. A start pulse signal "PHgr"s is applied to the gate electrode of the thyristor device T(1). Clock pulse signals "PHgr"1 and "PHgr"2 are applied to cathode electrodes of alternating thyristor devices T. The gate electrodes of the transfer thyristor devices T(1), T(2), . . . and the corresponding gate electrodes of the light-emitting thyristor devices are interconnected by means of wires G(1), G(2), . . . A write signal "PHgr"I is applied also to the cathode electrodes of the light-emitting thyristor devices L.
In the above-described circuit configuration, the thyristor devices T(1), T(2), . . . are sequentially turned on by means of the two clock pulse signals "PHgr"1 and "PHgr"2. In association with such turning-on action, the light-emitting thyristors L(1), L(2), . . . enter a state in which they can be turned on sequentially. If any one of light-emitting thyristor devices is turned on or enters a luminous state, the luminous intensity of the light-emitting thyristor device is determined by the amount of electric current to flow as a write signal "PHgr"I; that is, by resistance RI. An image can be written at arbitrary intensity. As can be seen from FIG. 12, the self-scan-type light-emitting array of such a configuration requires interconnection of only a total of six terminals per chip; that is, two power terminals and four signal terminals. Thus, the number of connections does not depend on the number of light-emitting devices mounted on one chip. Hence, in a case where 128 light-emitting devices, for example, are mounted per chip, the number of wires to be connected to a drive IC per chip can be reduced to one-twentieth those required for a related-art LED array chip.
By replacing a related-art LED array chip with the self-scan-type light-emitting array chip, a driver IC can be readily mounted on a substrate differing from that having light-emitting devices mounted thereon (see Japanese Patent Application Laid-Open No. 187981/1997). As shown in FIG. 13, a substrate 42 having light-emitting device array chips 40 mounted thereon is disposed opposite a rod lens array 44. A substrate 45 having the driver IC mounted thereon is separated from the substrate 42. The substrates 42 and 45 are connected together by means of a flexible printed circuit (FPC) film or sheet 47, The FPC substrate 47 is connected Lo the substrates 42 and 45 by means or soldering or through use of connectors. Such a construction can be said to be a method of reducing the width of the substrates and miniaturizing an optical write head more effectively than a method using the related-art LED array chin.
As mentioned above, in a case where the substrate having light-emitting devices mounted thereon is separated from the substrate having a driver IC mounted thereon, a certain number of wires, to be used for interconnecting the substrates are required. The wires are greater in number than those required when drawing wires, to the outside, from a substrate having light-emitting devices and a driver IC mounted thereon. The wires can be integrated simply by use of an FPC substrate. However, much space to be used for mounting connectors or space for soldering must be ensured on the substrate having light-emitting devices mounted thereon. Hence, the width of the substrate cannot be diminished much.
The present invention has been conceived to solve the drawback set forth and is aimed at providing a compact optical write head which substantially obviates a necessity of optical adjustment, thereby embodying a high-resolution electrophotographic printer.
(III) A light-emitting diode (LED) array has usually been used widely as a light-emitting device array. Each of the LED devices involves variations in the amount of light emission. Further, each of rod lenses involves variations in optical characteristic. These variations account for inconsistencies in density of an image. If a currently-available LED array is used in its present form, variations in density will exceed the allowable density limit of the LED. For this reason, the amount of light is corrected such that inconsistencies in density of an image fall within the allowable density limit of an LED, by means of changing drive conditions for each of LEDs. The amount of light is usually corrected in accordance with the following procedures. While the optical write head is separated from the printer, LEDs are illuminated one by one, and a light-receiving element is situated at a position where an image is to be formed thereby determining the distribution of light quantity over the head in its longitudinal direction. The thus-determined distribution of light quantity is recorded. A per-chip drive current to be supplied to LEDs or the period of illumination of the respective chip or LEDs is determined from the recorded light quantity distribution such that the light quantity distribution is flattened. At the time of actual use of the light-emitting array, the thus-determined drive conditions are employed.
However, in practice, an optical write head is assembled or used in the environment where ambient temperature changes. The positional precision of layout of light-emitting device array chips is influenced by thermal expansion of a glass epoxy substrate. Further, the positional precision of layout of a rod lens array is influenced by thermal expansion of glass-fiber-reinforced plastic (GFRP). Accordingly, there may be a case where the optical axis of a light-emitting array chip and the optical axis of a rod lens may deviate from an initially-adjusted position in the longitudinal direction of the print head. Inconsistencies in an image stemming from such deviation cannot be compensated by the electrical correction of light quantity set forth.
Even in a process of die-bonding light-emitting device array chips to a substrata, heating is required for setting a conductive adhesive in the course of a cooling operation for setting the thus-heated adhesive, residual stress develops between the chip and the substrate. The residual stress induces distortion in the substrate, thereby deteriorating the positional precision of the chip. Even a pitch between the chips encounters the same problem.
The present invention is aimed at solving the foregoing drawback, providing an optical write head having high reliability with respect to temperature variations, and realizing a high-resolution electrophotographic printer.
The present invention has been conceived to solve the problems set forth.
According to a first aspect of the present invention, there is provided an optical write head comprising a substrate, and a plurality of light-emitting device array chips arranged on the substrate in a straight line or in a staggered layout so as to oppose a gradient index rod lens array, each of the light-emitting array chips having a light-emitting device array, wherein the rod lens array, a substrate support member for supporting the substrate, and a driver circuit board are fixedly held by a support member.
Preferably, the support member and the substrate support member are formed from metallic material. Further, at least one of frames of the rod lens array to be bonded to a support member is preferably a glass plate.
Preferably, a plurality of adhesive injection holes are formed in a surface of the support member to which the rod lens array is to be brought into contact, arranged in a longitudinal direction of the rod lens array and formed so as to penetrate through the support member to a reverse side thereof. Alternatively, at least one slit of V-shaped cross section for injecting an adhesive is preferably formed in a portion of the surface of the support member to be brought into contact with the rod lens array, so as to extend in the longitudinal direction of the rod lens array, and a plurality of adhesive injection holes are formed in the slit so as to penetrate through the support member to a reverse side thereof.
Preferably, at least two positioning pins are provided at predetermined positions on the support member so as to come into contact with the substrate or the substrate support member. Alternatively, at least two eccentric pins capable of penetrating through the support member and rotating are preferably provided so as to come into contact with the substrate support member.
Preferably, the two eccentric pins are rotated, to thereby move the substrate support member remaining in contact with the eccentric pins and to adjust the distance between a light-emission section of the light-emitting device array and a light-incident end face of the rod lens array. Further, light-emitting array chips are preferably die-bonded to a substrate bonded to predetermined locations on the substrate support member while the position of the substrate is taken as a reference plane of the substrate support member.
Further, the substrate may be a flexible printed circuit substrate (FPC film or sheet). Moreover, the light-emitting device array may be a self-scan-type light-emitting device array (see, for example, U.S. Pat. No. 5,177,405).
According to a second aspect of the present invention, there is provided an optical write head in which light-emitting array chips are mounted directly on a flexible print circuit substrate (a FPC film or sheet). The FPC substrate is brought, in advance, into close contact with a member possessing rigidity. The FPC substrate is of multilayer type and preferably has a thickness of 30 to 50 xcexcm. As to the optical write head according to the present invention, a self-scan-type light-emitting device array is suitable as the light-emitting device array.
The optical write head according to the present invention is assembled in the following manner. A portion of the FPC substrate is bonded in advance to a member having rigidity. Next, light-emitting array chips are arranged on and directly die-bonded to the FPC substrate in the form of a straight line or in a staggered layout. Wire bonding pads provided on the light-emitting device array chips and wire bonding pads provided on the FPC substrate are electrically interconnected by means or wire bonding. Subsequently, the member having rigidity is mounted at a predetermined position on a support member having the rod lens array and the light-emitting array driver circuit mounted thereon.
The present invention proposes direct die-bonding of light-emitting array chips onto an FPC element (a flexible printed circuit film or sheet). As a result, a necessity of interconnecting the substrate having light-emitting devices mounted thereon and the driver circuit using a connector can be obviated. Since mounting of connectors to the substrate is obviated, the area of the substrate can be minimized correspondingly.
Accurately arranging and fixing chips on a flexible substrate is not easy. Further, wire-bonding chips onto resin which poses difficulty in propagation of ultrasonic waves is also difficult. For these reasons, a member having rigidity is brought into close contact with the reverse side of an area of the substrate at which chips are to be mounted. Further, the thickness of the FPC substrate is made as small as possible. In this state, the precision of positions of chips required by the optical wire head can be ensured by means of die-bonding or wire-bonding light-emitting array chips on the substrate. Further, electrical connection can be established easily. There can be prevented deformation of the area of the substrate in which chips have been mounted, which would otherwise be caused when the chips are mounted. Further, breakage or wire-bonded Au lines or rupture of chips can be prevented.
In a case where wires must be drawn to either side of a chip because of design or a light-emitting device array chip, wires can be drawn readily by use of an FPC substrate of multilayer type. By means of such a construction, the substantial area of the substrate can be reduced much further, thereby improving the freedom of design of an optical write head.
A member which has rigidity and is mounted on an FPC substrate is embodied as a single constituent component of an optical write head, thereby enabling very simple and highly accurate assembly of an optical write head.
According to a third aspect of the present invention, there is provided an optical write head comprising a flexible printed circuit substrate (a FPC film or sheet) remaining in close contact with a member having rigidity, and a plurality of light-emitting device array chips arranged on the FPC film or sheet in a straight line or in a staggered layout so as to oppose a gradient index rod lens array, each of the light-emitting array chips having a light-emitting device array, wherein the member having rigidity is a metallic member substantially equal in coefficient of thermal expansion to the rod lens array. Preferably, the member having rigidity is a metallic member substantially equal in coefficient of thermal expansion to the light-emitting device array chips.
Preferably, a frame of the rod lens array is formed from glass, and the metallic member is a nickel alloy or titanium Further, in a case where the light-emitting device array chips are formed from GaAs-based semiconductor, the member having rigidity can be made substantially equal in coefficient of thermal expansion to the light-emitting device array chips, as a result of use of the metallic material. Preferably, a self-scan-type light-emitting device array is used for the light-emitting device array.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. 2000-104786 (filed on Apr. 6, 2000), 2000-213005 (filed on Jul. 13, 2000), 2000-213006 (filed on Jul. 13, 2000), and 2000-310815 (filed on Oct. 11, 2000) which are expressly incorporated herein by reference in their entireties.